In order to more fully understand the invention disclosed and claimed herein, a brief discussion of polarization of light is considered helpful. Light generally travels in a transverse direction with electric vibrations perpendicular to the line of propogation of the light waves. Light is polarized linearly and horizontally when the electrical vibrations are horizontal; and when the vibrations are vertical, the light is considered to be polarized linearly and vertically. Thus, if a beam of light is passed through a first polarizer which divides the light into two components, one transmitted or passed through the polarizer while the other one is blocked, the remaining light has either horizontal or vertical polarization. If this polarized light subsequently is passed through a second polarizer maintained parallel to the first one, the polarized light all is transmitted through the second polarizer. If, however, the second polarizer is rotated, the amount of light passed through it decreases proportional to the amount of rotation of the second polarizer. When the polarizers are at right angles to one another, all of the light theoretically is absorbed by the second polarizer.
This phenomenon is employed to substantial advantage in the use of polarized sunglasses to substantially reduce the annoying effects of glare, since reflected sunlight (glare) has its polarization rotated ninety degrees or at right angles to direct sunlight. The most common polarized sunglasses are made of stretched plastic material which has long, thin, parallel chains of iodine or similar material embedded in it to permanently polarize it. Sunglasses made of this material have become very popular, but suffer from a number of inherent disadvantages. The plastics have a low hardness, and therefore, a poor scratch resistence; so that unless a great deal of care is taken to avoid scratching them, lenses made of such plastics rapidly deteriorate to the point where they are unusable or should not be used by the wearer. In addition, these plastics have a low refractive index which prevents manufacture of prescription polarized sunglasses from them.
Prescription sunglasses have been developed using photochromic glasses, which include submicroscopic crystals of silver halides, such as silver chloride, silver chromide, or slver iodide, which become darker in color when the glass is subjected to actinic radiation, but which regain their original color (or clarity) when the radiation is removed or reduced. Such a photochromic glass is disclosed in the patent to Armistead et al, U.S. Pat. No. 3,208,860. A later patent to Hares et al, U.S. Pat. No. 4,190,451, discloses a photochromic glass which is described as also having the capability of being either thermally tempered or chemically strengthened to comply with present regulations in existence for use in opthalmic applications. The glasses disclosed in both of these patents, however, are not polarized glasses, but simply exhibit the characteristics of becoming darker when exposed to actinic radiation, and then fading or returning to their original color when such radiation is removed.
The formation of photochromic glass of the type disclosed in the Armistead and Hares et al patents requires the deliberate introduction of silver halides into the glass along with small amounts of reducing agents. Bulk processing of the glass takes place since the silver halide crystals and the reducing agents are uniformly dispersed throughout the glass.
An improvement in the photochromic glasses described above is disclosed in the patents to Simms, U.S. Pat. No. 3,892,582, and 3,920,463. These patents both disclose processes for permanently tinting photochromic glass by heating the photochromic material in a reducing atmosphere, and while it is at an elevated temperature, irradiating it with ultraviolet irradiation. The change in the tint of the photochromic material is caused by the heating and is emphasized or darkened by the subsequent ultraviolet irradiation. The formation of the glasses described in the Simms patents includes the introduction of silver halides into the glass batch in a manner similar to the production of the photochromic glasses described in the Armistead and Hares et al patents. The improvement is in the introduction of a permanent overriding tint of varying intensity coupled with the photochromic characteristics of the glasses.
An effort to combine photochromic characteristics with polarization in ophthalmic lenses, and the like, is disclosed in the patent to Araujo, et al, U.S. Pat. No. 3,653,863. The glass described in the Araujo patent is made by introducing crystalline silver halide into the glass batch along with a small amount of low temperature reducing agents in the batch. When the batch is subjected to heat, the reducing agents are catalyzed and operate to reduce the silver halides to metal. Submicroscopic droplets are formed; and as the glass remains at elevated temperatures after the reduction, these droplets agglomorate to form larger balls or masses of silver (silver halide) droplets. The glass then is stretched to elongate the particles causing elongated fibrils all aligned in the same direction to be formed throughout the glass bulk.
Because a bulk process is employed in the Araujo patent, it is somewhat difficult to control the transmission characteristics of the completed lens. For example, it is possible for several fibrils of the elongated droplets to be aligned with or nearly aligned with one another throughout the thickness of the lens. The only fibrils, however, required for the polarization effect are the outer-most ones where the light enters the lens. The remaining fibrils simply reduce the transmission of light through the lens due to scattering and the like. This difficulty is inherent in the bulk effect technique which is employed in the Araujo method. The aligned lines of polarizing material are noncontributors to the polarizing effect and simply contribute to transmission losses. All of this is particularly significant if the intent is to manufacture a "clear", high transmission prescription ophthalmic lens or a photochromic lens where ranges of transmission have to be carefully controlled as the photochromic lens in a sunglass.
Accordingly, it is desirable to manufacture polarized ophthalmic lenses of high quality without the above disadvantages.